SCS PTC-IIex, Manual

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15 Basics
•
Longpath option (ARQ links over the long path possible).
•
Reliable QRT acknowledgment from both sides (not just a simple time-out).
•
Fast and reliable change of data direction.
•
High performance read function without additional software.
•
Capable of running as a stand-alone controller i.e. independence from IBM compa-
tible PC’s.
5. Full compatibility with the older protocol.
•
Automatic switching between Level-I and Level-II at contact initialization. (The
user should be able to use the usual command syntax
C CALLSIGN
to start a
PACTOR contact, without having to worry about the other station's system level.)
6. A bandwidth of less than 500 Hz at -50 dB, so that operation within 500 Hz channels
is possible.
7. Constant bandwidth, irrespective of the actual effective transmission speed.
8. The acknowledgment signal (CS) should be equally as robust, or even more so, than
the actual data signal.
All the above points are fulfilled with PACTOR-II, and not only those. PACTOR-II uses
an extended and better on-line data compression system known as Markow coding. A
reliable and automatic frequency correction, adaptive cycle length, and many other useful
features are also incorporated.
15.3 Basics of the PACTOR-II protocol
15.3.1 General
The PACTOR-II protocol (PT-II) is essentially based on the Level-I standard, consisting
of a synchronous half-duplex ARQ protocol. New, however, is the ability to choose four
different speed steps, so that a greatly improved adaptability is obtained. The modulation
system used for PT-II is based on DPSK (differential phase shift keying - see below).
which leads to a very narrow spectrum, practically independent of the data rate. The
robustness of the DPSK modulation qualifies itself noticeably higher at lower information
speeds in comparison to FSK. In order to effect a further step towards robustness, PT-II
uses high performance convolutional coding, that is evaluated with a real Viterbi decoder
in the data receiver (see below). The high correction capability of the decoder allows not
only links with extremely weak or noisy signals, but also, with more normal signals,
enables short error bursts, or fadeouts, to be entirely ignored, and a repetition of that
packet is not required. This is especially important with PT-II, as the new protocol allows
switching to a triple cycle length if there is enough data in the transmit buffer. The
relatively long resultant data packet would be very prone to impulse errors from clicks or
atmospherics (QRN), if not for the highly effective error correction designed.
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